This is almost certainly Ukonline's doing. I first noticed this in October (dcc and most priv msgs basically rendered useless) and have exactly the same issues with irssi on cygwin and on two separate Linux machines. The support staff are useless droids and there seems to be no way of telling whether or not this is intentional blocking.
Would suggest waiting for the new
OFCOM migration rules to kick in on Feb 14th and switching to a different provider (Plusnet, Zen, etc).